This invention relates to an unburned carbon-containing refractory suitable for lining a container for molten metal and, more particularly, to an unburned carbon-containing refractory suitable for lining a molten metal container, which can reduce the outer steel shell temperature of a molten metal container thereby making it possible to prevent deformation of the outer steel shell, to suppress a reduction in molten steel temperature, and the like, and a molten metal container.
Carbon-containing refractories containing carbon, e.g., graphite, such as magnesia-carbon brick and alumina-carbon brick, have high durability owing to their excellent resistance to corrosion and oxidation and are now made great use of for wear lining various containers for transportation or for treatments (RH degassing, LF treatment, etc.), such as a torpedo car, a hot metal ladle, a converter, a ladle, a degasser, a holding furnace, an electric furnace, and the like, particularly those for iron and steel making.
In molten steel processing such as RH degassing and LF treatment, however, since molten steel holds in the container for a long time, the high thermal conductivity of carbon has given rise to such problems that the outer steel shell temperature increases, which can result in deformation of the outer steel shell or increase of thermal dissipation.
It will also increase the temperature drop of the molten metal while transported or processed, which results in a great loss of energy.
Further, the outer steel shell is apt to undergo deformation at elevated temperatures so that the durability of the container shell is reduced. The bottom of the container is similarly exposed to high temperature so that casting troubles due to deformation of the slide valve apparatus occur.
It has been one of the most important subjects to enhance energy efficiency to the possible maximum in iron and steel manufacture and nonferrous metal manufacture that are high-energy-consuming industries.
In regard to these problems, the following techniques have been disclosed to date. Taikabutsu (Refractories), vol. 149, No. 10, pp. 574-575 (1997) discloses application of a microporous heat insulating material (thickness: 3 mm) to the side walls of a ladle to drop the outer steel shell temperature by about 55xc2x0 C. However, application of back-up insulation to the side walls increases the cost of refractory lining. Having a large number of pores inside, the heat insulating material is not deemed to have high strength. Therefore the structure of the heat insulating material is liable to compressive deformation due to, for example, thermal expansion of the refractory upon receipt of molten steel to reduce its heat insulating properties and has difficulty in manifesting its effects continuously. The duration in which the heat insulating material works effectively is usually about equal to or twice the duration of wear refractory. When the heat insulating materials are exchanged, it is necessary to dismantle the back lining refractory. As a result, the back lining refractory, whose duration is usually 5 to 8 times that of wear refractory, comes to have a reduced duration to the same level of, or about twice as durable at the most as, wear refractory.
Further, where the heat insulating material is used, the heat that should have escaped through the outer steel shell would be confined in the refractory to increase the temperature of the whole refractory, causing reduction of refractory life (particular of wear refractory).
Taikabutsu (Refractories), vol. 141, No. 7, pp. 365-370 (1989) proposes a method for improving the life of an outer steel shell of a converter, in which a blast cooling system is adopted to the wall and the lower corn of a converter, and a water cooling pipe system to the upper corn. However, this method cannot be applied to a ladle of LF processing from the standpoint of heat loss.
For the purpose of stress relaxation of a converter, etc. in high temperature, the generated stress is reduced by designing the lining taking the expansion of MgOxe2x80x94C brick into consideration. However, in case where the furnace is operated basically at high outer steel shell temperature, the outer steel shell temperature increases, and the problems of deformation and heat loss remain unsolved.
As stated above, the conventional techniques for reducing the outer steel shell temperature and thereby preventing deformation of the shell include introduction of a water cooling or air cooling system, application of a heat insulating material on the back side of the back lining brick, reduction of hot stress generation, and the like. However, they are not satisfactory from the standpoint of cost, duration of containers, and energy saving.
The present invention has been made in the light of the above-described problems. It is an object of the invention to provide a carbon-containing refractory suitable for lining a molten metal container which prevents deformation of the outer steel shell of the container, improve the furnace life, suppress a reduction in molten steel temperature, minimize energy losses, and secure the life of wear lining, and to provide a molten metal container lined with the carbon-containing refractory, which is capable of reducing energy losses and has stable durability.
The term xe2x80x9cmolten metal containerxe2x80x9d as used herein is intended to include various containers for transportation or for treatments (RH vacuum degassing equipment, LF treatment, etc.) of molten metal, such as a torpedo car, a hot metal ladle, a converter, a ladle, a degasser, a holding furnace, an electric furnace, and the like.
The present inventors have conducted extensive investigations on techniques for reducing energy losses from a molten metal container and techniques for not reducing durability such as resistance to corrosion, spalling, oxidation, etc. and, as a result, completed the present invention.
The present invention will be further described in detail.
Carbon-containing refractories which are currently employed for wear lining various melting furnaces have respective proper carbon contents for securing satisfactory durability in conformity to the respective conditions of use. That is, carbon-containing refractories having different carbon contents are used in accordance with the melting furnaces to which they are applied.
The recent development of techniques of production and material designing of carbon-containing refractories has provided high-density carbon-containing refractories.
High-density carbon-containing refractories have satisfactory durability against corrosion and oxidation but, on the other hand, allow the inner temperature to rise because of their high thermal conductivity. It follows that the outer steel shell temperature of the melting furnace increases, and thermal dissipation from both the melting furnace and the molten metal increases.
Under the present circumstances the high-density carbon-containing refractories now in use for various melting furnaces have a thermal conductivity xcex(W/mxc2x0 C.) ranging from about 0.8X+8 less than xcex less than 0.8X+16, wherein X is the carbon content of the refractories, while dependent on the kind of the refractory aggregate used.
The inventors have studied techniques of reducing energy losses from a molten metal container and found, as a result, that the object is accomplished by a carbon-containing refractory which contains 30% by weight or less, preferably 1% to 20% by weight, of a carbon raw material comprising at least graphite, and whose carbon content X wt % and whose thermal conductivity xcex(W/mxc2x0 C.) satisfy the relationship xcexxe2x89xa60.8X+7. The present invention has thus been reached.
The inventors have also studied techniques for reducing energy losses from a molten metal container without reducing the durability such as resistance to corrosion, spalling, oxidation and the like and recognized, as a result, that the object is accomplished by a carbon-containing refractory which contains 30% by weight or less, of a carbon raw material comprising at least graphite, whose carbon content X wt % and whose thermal conductivity xcex(W/mxc2x0 C.) satisfy the relationship: xcexxe2x89xa60.8X+7, and which contain 20 parts by weight or more of a refractory aggregate having an apparent porosity of 10% or smaller and a greater size than 1 mm per 100 parts by weight of the total of the refractory aggregate and the carbon raw material. The present invention has thus been reached.
These techniques have now provided a carbon-containing refractory which is highly dense and yet capable of reducing energy losses from a molten metal container to which it is applied.
In particular, it has been found that the carbon-containing refractory containing 1% to 20% by weight of a carbon raw material comprising at least graphite has its thermal conductivity reduced by 10% to 30% or even more as compared with a conventional carbon-containing refractory having an equal carbon content and that a molten metal container lined therewith has the back side temperature of its carbon-containing refractory brick reduced by 100 to 150xc2x0 C. and its outer steel shell temperature reduced to 500xc2x0 C. or lower and thus established a further improved invention.
That is, the gist (particulars specifying the invention) of the invention lies in:
(1) an unburned carbon-containing refractory containing a refractory aggregate and 30% by weight or less of a carbon raw material comprising at least graphite, which is characterized in that the carbon content X (% by weight) and the thermal conductivity xcex(W/mxc2x0 C.) of the refractory satisfy the relationship: xcexxe2x89xa60.8X+7, and the refractory contains 20 parts by weight or more of a refractory aggregate having an apparent porosity of 10% or smaller and a greater size than 1 mm per 100 parts by weight of the total of the refractory aggregate and the carbon raw material.
The gist (particulars specifying the invention) of the invention also lies in:
(2) an unburned carbon-containing refractory containing a refractory aggregate comprising 50% by weight or more of a magnesia raw material and 30% by weight or less of a carbon raw material comprising at least graphite, which is characterized in that the carbon content X (% by weight) and the thermal conductivity xcex(W/mxc2x0 C.) of the refractory satisfy the relationship: xcexxe2x89xa60.8X+7, and the refractory contains 20 parts by weight or more of a refractory aggregate having an apparent porosity of 6% or smaller and a greater size than 1 mm per 100 parts by weight of the total of the refractory aggregate and the carbon raw material.
The gist (particulars specifying the invention) of the invention also resides in:
(3) an unburned carbon-containing refractory containing a refractory aggregate comprising 50% by weight or more of an alumina raw material whose alkali content is not more than 1% by weight and 30% by weight or less of a carbon raw material comprising at least graphite, which is characterized in that the carbon content X (% by weight) and the thermal conductivity xcex(W/mxc2x0 C.) of the refractory satisfy the relationship: xcexxe2x89xa60.8X+5, and the refractory contains 20 parts by weight or more of a refractory aggregate having an apparent porosity of 10% or smaller and a greater size than 1 mm per 100 parts by weight of the total of the refractory aggregate and the carbon raw material.
The unburned carbon-containing refractory according to the invention as set forth above is characterized in that:
(4) the carbon raw material comprising at least graphite is present in an amount of 1% to 20% by weight.
The gist (particulars specifying the invention) of the invention also resides in:
(5) a molten metal container characterized in that any of the above-described unburned carbon-containing refractory is used at least partly as the wear lining.
The gist (particulars specifying the invention) of the invention also lies in:
(6) a molten metal container characterized in that any of the above-described unburned carbon-containing refractory is used at least partly as the wear lining, and the outer steel shell temperature is 500xc2x0 C. or lower.
The gist (particulars specifying the invention) of the invention also lies in:
(7) a molten metal container characterized in that any of the above-described unburned carbon-containing refractory is used at least partly as the wear lining without having back-up insulation on the back side thereof, and the outer steel shell temperature is 500xc2x0 C. or lower.
The gist (particulars specifying the invention) of the invention also lies in:
(8) a molten metal container characterized in that any of the above-described unburned carbon-containing refractory is used at least partly as the wear lining, the carbon content X (% by weight) and the thermal conductivity xcex(W/mxc2x0 C.) of the refractory satisfy the relationship: xcexxe2x89xa60.8X+7, and the outer steel shell temperature is 500xc2x0 C. or lower.
The gist (particulars specifying the invention) of the invention also consists in:
(9) a molten metal container characterized in that any of the above-described unburned carbon-containing refractory is used at least partly as the wear lining without having back-up insulation on their back side, the carbon content X (% by weight) and the thermal conductivity xcex(W/mxc2x0 C.) of the refractory satisfy the relationship: xcexxe2x89xa60.8X+7, and the outer steel shell temperature is 500xc2x0 C. or lower.